High precision, low-noise amplifiers are needed where accurate amplification of very low level signals is required. Typical low level applications include thermocouple and strain gauge measurement systems which typically have very low signal levels. The signal to be measured may be in the range of 20-60 millivolts. Often it is desirable to have measurement errors as low as one unit of error in 40,000. This requires signal resolution less than a microvolt.
A major source of amplifier error is in offset voltage. The offset voltage is the voltage difference between the non-inverting and the inverting terminals of the amplifier, required to provide a zero volt output signal. In low cost commercial amplifiers, these offset voltages can be several millivolts. Precision operational amplifiers usually maintain the offset voltage at about 100 microvolts. The best of them can achieve a 10 microvolt offset voltage, and chopper-stabilized operational amplifiers commonly have offset voltages of 5 microvolts or less.
Accordingly, chopper-stabilized amplifiers have been the amplifier of choice for low level, precision gain applications. The offset temperature drift of chopper amplifiers is almost nonexistent making them very desirable. However, these amplifiers have long recovery times from overload, typically operate at low speeds, and have clock and 1/f noise that provide serious drawbacks.
The precision operational amplifier of this invention has all of the advantages of chopper stabilized amplifiers yet virtually eliminates 1/f noise. Moreover, the precision amplifier of this invention recovers quickly from overload and has a respectable bandwidth for amplifying signals of moderate speed. The amplifier uses a novel technique for reducing offset voltage without increasing the noise of the amplifier.